Method for monitoring a CMP polishing method and arrangement for a CMP polishing method

ABSTRACT

The invention relates to a method for monitoring a CMP polishing method, a substrate being fixed in a mount, a polishing pad being fixed on a plate, a surface of the polishing pad being operatively connected to a surface of the substrate, the polishing pad and the substrate being moved relative to one another, so that material is removed from the surface of the substrate. A measuring device is provided, which is operatively connected to the surface of the polishing pad, the measuring device detecting the surface constitution of the polishing pad and generating a measurement signal dependent on the surface constitution of the polishing pad. The measurement signal is compared with corresponding reference values for the purpose of identifying a change in the material of the surface of the substrate during the CMP polishing method.

CLAIM FOR PRIORITY

This application claims the benefit of priority to German Application No. 10 2004 058 133.9, filed Dec. 2, 2004.

TECHNICAL FIELD OF THE INVENTION

The invention relates to a method for monitoring a CMP polishing method and to an arrangement for carrying out a CMP polishing method.

BACKGROUND OF THE INVENTION

CMP polishing methods are essential and important methods in the field of semiconductor technology, in particular in the fabrication of integrated circuits, such as e.g. memory chips. The CMP method, that is to say chemical mechanical polishing, is a method for processing a substrate surface. The CMP method is used in the fabrication of a memory chip, for example, to level or remove a substrate surface. During the CMP method, a rotating polishing surface is prestressed against the surface of the substrate to be polished. By means of a relative movement between the polishing surface and the substrate, material is moved from the surface of the substrate. During polishing, it is possible to introduce a polishing fluid with both chemical and mechanically abrasive agents between the substrate and the polishing surface, in order to improve the polishing process.

An important parameter for a precise CMP polishing method is the identification of a defined end point of the polishing process at which the substrate has the desired surface.

DE 697 11 811 T2 discloses an end point detector for a chemical mechanical planarizing system, in which the end point of the polishing process is identified by an abrupt change in the coefficient of friction at the contact area between the polishing pad and the substrate. The change in the coefficient of friction is also accompanied by an alteration of the torque desired for the desired speed of the polishing pad. Consequently, an end point of the polishing process can be identified by monitoring the change in torque. Furthermore, the U.S. Pat. No. 6,257,953 B1 discloses a method and a device for controlling a polishing method. In the case of this device, the mount of the polishing pad has a sensor, which detects a change in force and/or torque between the polishing pad and the wafer, so that a control of the polishing method is carried out on account of the change in force and/or torque.

Furthermore, the article by Norm V. Gitis “Tribology Issues in CMP, Semiconductor Fabtech, 18th edition, pages 125 to 128, discloses providing a measuring arrangement in a CMP process, the surface constitution of the polishing pad being detected by means of said measuring arrangement. The measurement signals of the measuring arrangement are used to start a conditioning process for the polishing pad. Moreover, the measurement signal of the measuring arrangement is used to detect wear of the polishing pad.

SUMMARY OF THE INVENTION

The invention provides an improved method and an improved arrangement for controlling a CMP method.

In one embodiment according to the invention, there is a measurement signal of the measuring device that detects the surface constitution of the polishing pad is used for identifying the change in the material constitution of the wafer. Experiments have shown that the measurement signal of the measuring device can also be used for controlling the polishing method itself. Consequently, the signal which is provided by the measuring device and which is used in the prior art for assessing the surface property of the polishing pad can at the same time be used for assessing the surface constitution of the wafer and, in particular, for controlling the polishing method.

In another embodiment, the measuring device detects a force effect between the polishing pad and the measuring device. Instead of the force effect, it is also possible, in a further advantageous embodiment, to detect a friction effect between the polishing pad and the measuring device, in order to identify a change in the material of the surface of the wafer. The use of the force effect and/or the friction effect between the polishing pad and the measuring device affords the advantage that simple detection is possible. Torque or force sensors that provide a reliable signal may be used for this purpose.

In still another embodiment, a change in the surface constitution of the wafer is identified if the measurement signal of the measuring device changes by a defined value. Consequently, an end point of a layer on the wafer surface or the beginning of a new layer on the wafer surface can reliably be identified. A precise control of the CMP process can thus be carried out.

In yet another embodiment, reference values corresponding to a defined material change are stored for the measurement signal and/or the change in the measurement signal. Consequently, a material change in the surface of the wafer can be identified by means of a simple comparison. It is thus possible to precisely identify the material change even in the case of a wide variety of material compositions of the substrate surface. In another embodiment, the measurement signal of the measuring device is used for the control of the CMP process and, in particular for the end point identification and for ending the CMP process. Reliable control of the CMP process is thus possible.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is explained in more detail below with reference to the figures, in which:

FIG. 1 shows an arrangement for carrying out a CMP polishing method.

FIG. 2 shows a partial cross section through a substrate before a polishing process and after a polishing process.

FIG. 3 shows a diagram with measurement signals of the measuring device.

DETAILED DESCRIPTION OF THE INVENTION

FIG. 1 shows a schematic illustration of an arrangement for carrying out a CMP polishing method. The arrangement has a polishing pad holder 1, on which a polishing pad 2 is fixed. The polishing pad holder 1 is connected to a drive unit 4 via a drive shaft 3. The polishing pad holder 1 is mounted such that it can be rotated in a longitudinal axis of the drive shaft 3 by means of the drive unit 4.

A substrate 5 in the form of a wafer, for example a silicon wafer, bears on the polishing pad 2. The substrate 5 is fixed to a substrate holder 6, which is likewise connected to the drive unit 4 by means of a second drive shaft 7. The drive unit 4 is designed in such a way that the substrate holder 6 is both rotatable about a center axis of the second drive shaft 7 and displaceable parallel to the surface of the polishing pad 2. Moreover, the polishing pad holder 1 and the substrate holder 6 can be prestressed relative to one another. As a result, it is possible to set the frictional force between the polishing pad and the substrate and, consequently, to influence the speed of the removal process.

Furthermore, a measuring device 8 is provided, which has a contact plate 9, a sensor 10, an evaluation unit 11 and a data memory 12. The sensor 10 and the contact plate 9 are fixed to a second drive unit 13, by means of which the contact plate 9 and the sensor 10 can be moved in the direction of the polishing pad 2. The evaluation unit 11 is connected to the sensor 10 via signal lines 14. Moreover, the evaluation unit 11 is connected to the data memory 12 via a data line 15. In one preferred embodiment, the evaluation unit 11 is connected to the drive unit 4 via a control line 16. Furthermore, a supply line 17 is provided for feeding polishing fluid on to the surface of the polishing pad 2.

The functioning of the CMP polishing arrangement in accordance with FIG. 2 is explained below. At the beginning of the CMP process, the substrate 5 is placed onto a polishing pad surface 19 by a substrate surface 18 by means of the drive unit 4. A desired prestress of the substrate 5 against the polishing pad 2 may preferably be set in this case. Moreover, polishing fluid is applied to the polishing pad surface 19 via the supply line 17. The polishing fluid may contain chemical additives and/or mechanical particles which advantageously influence the polishing process.

The polishing pad holder 1 is subsequently caused to effect a rotary movement which is oriented according to the center axis of the drive shaft 3. Moreover, the substrate holder 6 is caused to effect a rotary movement according to the center axis of the second drive shaft 7. The substrate holder 6 is additionally caused to effect a movement in which the substrate 5 is moved back and forth between the midpoint and the edge region of the polishing pad 2.

Furthermore, the second drive unit 13 presses the contact plate 9 against the polishing pad surface 19 with a defined force. Moreover, a force and/or torque acting between the contact plate 9 and the polishing pad surface 19 is detected by means of the sensor 10, which is formed as a force and/or torque sensor in the exemplary embodiment illustrated, and is forwarded to the evaluation unit 11 via sensor lines 14. The sensor 10 preferably detects the frictional force between the contact plate 9 and the polishing pad surface 19.

Depending on the embodiment chosen, it is not necessary for the contact plate 9 to bear on the polishing pad surface 19 throughout the CMP polishing method, rather it suffices, in one preferred embodiment, if the contact plate 9 is brought into contact with the polishing pad surface 19 at defined time intervals.

The measurement signals communicated to the evaluation unit 11 by the sensor 10 are compared with defined reference values by the evaluation unit 11. The defined reference values are stored in the data memory 12 and correspond for example to friction values and/or torque values of predefined substrate surfaces, that is to say predefined material compositions of the substrate surface and/or predefined end points of the CMP process at which the CMP process has generated a desired substrate surface and the CMP process is ended. In a further preferred embodiment, the friction values and/or torque values are not stored as reference values, rather their temporal change is stored as reference values.

Preferably, the evaluation unit 11 compares the measurement signals supplied by the sensor 10 with the stored reference values continuously during the CMP process. If the comparison reveals that the measurement signal of the sensor 10 corresponds to a stored reference value, then a corresponding material composition of the substrate surface 18 is identified by the evaluation unit 11. The identification of the predefined material composition of the substrate surface is then preferably used to control the CMP process. By way of example, the control of the CMP process may consist in altering parameters of the CMP process, such as e.g. the composition of the polishing fluid, the prestress of the substrate 5 against the polishing pad 2 and/or the movement of the substrate holder 6 and/or the movement of the polishing pad holder 1.

In one preferred embodiment, in which the measurement signal supplied by the sensor 10 corresponds to an end point of the CMP process, the CMP process is ended by the evaluation unit 11 by means of a control signal to the drive unit 4. For this purpose, the drive unit 4 ends the driving of the drive shaft 3 and of the second drive shaft 7 and additionally lifts the substrate 5 off the polishing pad 2, so that the substrate 5 can be exchanged for a new substrate.

FIG. 2 a shows a substrate 5 before a CMP polishing method. The substrate 5 has a stepped surface under which is formed a structure with polysilicon pillars 20 which are each covered with a nitride layer 21. The entire structure is overfilled with a silicate glass 22. For this exemplary embodiment, the data memory 12 stores an end reference value for the measurement signal of the sensor 10 for the friction value between the contact plate 9 and the polishing pad surface 19 if the surfaces of the polysilicon pillars 20 are reached during the CMP removal process.

The substrate 5 illustrated in FIG. 2 a is fixed to the substrate holder 6 and subjected to a CMP polishing method in accordance with the method described previously. In this case, the operative connection between the contact plate 9 and the polishing pad 2 is continuously sensed by means of the measuring device 8. The measurement signals detected by the sensor 10 are forwarded to the evaluation unit 11. The evaluation unit 11 compares the measurement signals communicated by the sensor 10 with the end reference value stored in the data memory 12. If the comparison reveals that the measurement signal communicated by the sensor 10 corresponds to the end reference value, then the CMP polishing process is ended. In the exemplary embodiment illustrated, the end reference value that is stored in the data memory 12 is the frictional force which occurs between the polishing pad 2 and the contact plate 9 if the substrate 5 has been removed as far as the surfaces of the polysilicon pillar 20, as is illustrated in FIG. 2 b. In this case, both the absolute friction value and a temporal derivative of the friction value are stored in the data memory 12, which occur upon the transition from the nitride layers 21 to the polysilicon pillars 20. Consequently, the method described identifies, on account of the comparison of the measurement signal and/or the temporal change in the measurement signal, that the substrate 5 has been removed by means of the polishing method in accordance with FIG. 2 b.

FIG. 3 shows a diagram for the measurement signal of the sensor 10, which was detected during the CMP process in which the substrate 5 of FIG. 2 a was removed as far as the layer thickness illustrated in FIG. 2 b.

The friction signal detected by the sensor 10 is plotted against time t in the diagram of FIG. 3. Moreover, a chemical sensor signal (EP signal) representing the chemical composition of the polishing fluid is plotted in parallel against time. The chemical sensor signal is detected by means of a corresponding chemical sensor and indicates a rise if the nitride layers 21 have been completely removed and the polysilicon layers 20 have started to be removed. The CMP method starts at a zeroth instant T0. In this case, the measurement signal of the sensor 10 first rises sharply and then falls approximately asymptotically to an intermediate value over time. The intermediate value is subsequently approximately maintained for a defined time duration until the first instant T1, when a renewed sharp fall in the friction signal occurs. This renewed sharp fall indicates the change during the polishing method from the nitride layers 21 to the polysilicon pillars 20. Consequently, the reaching of the polysilicon pillars 20 in accordance with FIG. 2 b can be identified in such a way that, after reaching an intermediate value, the measurement signal representing a friction signal once again has a defined temporal decrease, both the intermediate values and the temporal change of the friction signal being stored in the data memory 12 for the purpose of comparison and for the purpose of identifying the assigned surface situation in accordance with FIG. 2 b.

Instead of the friction signal explicitly illustrated in FIG. 3, it is also possible for a torque signal to be detected. The method according to the invention and the device according to the invention can be used for a wide variety of types of substrates in order to identify a defined material change at the surface of the substrate during the CMP process. For this purpose, corresponding comparison values, which have been determined experimentally, by way of example, are stored in the data memory. 

1. A method for monitoring a CMP polishing method, comprising: fixing a substrate in a mount; fixing a polishing pad on a plate, a surface of the polishing pad operatively connected to a surface of the substrate; moving the polishing pad and the substrate relative to one another, so that material is removed from the surface of the substrate; providing a measuring device, which is operatively connected to the surface of the polishing pad; and generating a measurement signal dependent on the surface constitution of the polishing pad, using the measuring device wherein the measurement signal is used for identifying a change in the material of the surface of the substrate during the CMP polishing method.
 2. The method as claimed in claim 1, wherein the measuring device detects a force effect between the polishing pad and the measuring device.
 3. The method as claimed in claim 1, wherein the measuring device detects a friction effect between the polishing pad and the measuring device.
 4. The method as claimed in one of claims 1, wherein a defined change in the measurement signal is identified as a change in the surface material of the substrate.
 5. The method as claimed in one of claims 1, wherein the measurement signal and/or a change in the measurement signal is compared with a reference value, and wherein a material change in the surface of the substrate is identified depending on the comparison.
 6. The method as claimed in one of claims 1, wherein the measurement signal and/or the change in the measurement signal is compared with a reference value corresponding to an end point of the CMP process, and wherein in the event of the measurement signal and/or the change in the measurement signal matching the reference signal, an end point for the CMP process is identified and the CMP process is ended.
 7. An arrangement for carrying out a CMP polishing method, comprising: a substrate mount for holding a substrate; a polishing pad mount for holding a polishing pad; a drive, by means of which the substrate mount and the polishing pad mount can be moved relative to one another, the substrate mount and the polishing pad mount being formed that the polishing pad can be operatively connected to a surface of the substrate; a measuring device, which can be operatively connected to the polishing pad, the measuring device having a sensor, by means of which the surface constitution of the polishing pad can be detected, the measuring device configured to generate a measurement signal dependent on the surface constitution of the polishing pad; and an evaluation unit is provided, which is connected to the sensor, wherein the evaluation unit is connected to a data memory, reference values for the measurement signal of the sensor, which are assigned to a surface constitution of the substrate, are stored in the data memory, the measurement signal can be fed to the evaluation unit, and the evaluation unit is designed to identify a surface constitution and/or a surface change of the substrate by way of a comparison between the measurement signal and a reference value.
 8. The arrangement as claimed in claim 7, wherein the sensor detects a friction value between the polishing pad and the measuring device, and the evaluation unit identifies a surface constitution and/or a surface change of the substrate from the friction value and the corresponding reference value stored in the data memory.
 9. The arrangement as claimed in claim 7, wherein the measuring device detects, as measurement signal, a change in the friction between the polishing pad and the measuring device, and the evaluation unit is designed to identify a surface constitution and/or a surface change of the substrate from the change in the friction and the corresponding reference value stored in the data memory.
 10. The arrangement as claimed in claim 7, wherein reference values for the measurement signal, which correspond to an end point of the CMP process, are stored in the data memory, the evaluation unit is connected to the drive of the substrate and polishing pad mounts, and the evaluation unit is designed to identify an end point for the CMP process on account of the comparison between the measurement signal and the reference value and to end the CMP process.
 11. A method for monitoring a CMP polishing method, comprising: fixing a substrate in a mount; fixing a polishing pad on a plate; operatively connecting a surfaced of the polishing pad to a surface of the substrate, the polishing pad and the substrate being moved relative to one another, so that material is removed from the surface of the substrate; providing a measuring device, which is operatively connected to the surface of the polishing pad, the measuring device generating a measurement signal dependent on the surface constitution of the polishing pad, the measurement signal being used for identifying a change in the material of the surface of the substrate during the CMP polishing method, wherein the measuring device detects a force effect or a friction effect between the polishing pad and the measuring device.
 12. An arrangement for carrying out a CMP polishing method, comprising: a substrate mount for holding a substrate; a polishing pad mount for holding a polishing pad; a drive, by means of which the substrate mounts and the polishing pad mount can be moved relative to one another, the substrate mount and the polishing pad mount being formed that the polishing pad can be operatively connected to a surface of the substrate; a measuring device, which can be operatively connected to the polishing pad, the measuring device having a sensor, by means of which the surface constitution of the polishing pad can be detected, the measuring device configured to generate a measurement signal dependent on the surface constitution of the polishing pad; an evaluation unit, which is connected to the sensor, wherein the evaluation unit is connected to a data memory, reference values for the measurement signal of the sensor, which are assigned to a surface constitution of the substrate, are stored in the data memory, the measurement signal can be fed to the evaluation unit, and the evaluation unit is designed to identify a surface constitution and/or a surface change of the substrate by way of a comparison between the measurement signal and a reference value, the sensor detects, as measurement signal, a friction value between the polishing pad and the measuring device, and the evaluation unit identifies a surface constitution and/or a surface change of the substrate from the friction value and the corresponding reference value stored in the data memory. 